1. Field of the Invention
The present invention relates to a group III-V type nitride semiconductor device, and particularly to reduction of lattice mismatch between a substrate material and a nitride semiconductor formed thereon.
2. Description of the Related Art
The typical substrate material for group III-V type nitride semiconductor devices conventionally includes Al.sub.2 O.sub.3, SiC and the like. Recently, an attempt has been made to use LiAlO.sub.2 and LiGaO.sub.2 as substrate materials for epitaxial growth of a nitride semiconductor layer, as described in "GROWTH AND OPTICAL PROPERTIES OF GaN GROWN BY MBE ON NOVEL LATTICE-MATCHED OXIDE SUBSTRATES", J. F. H. Nicholls et al., Mat. Res. Soc. Symp. Proc., Vol. 395, 1996, Materials Research Society, pp. 535-539. Furthermore, an attempt has been made to use MgAl.sub.2 O.sub.4 as a substrate material, as described in "InGaN Multi-quantum-well structure laser diodes grown on MgAl.sub.2 O.sub.4 substrates", S. Nakamura et al., Appl. Phys. Lett., 68 (1996) pp. 2105-2107.
When Al.sub.2 O.sub.3 is used as a substrate material, however, the lattice mismatch between e.g. GaN and the substrate reaches as high as at least 10%. When a substrate of SiC is used, GaN and AlN cause lattice mismatches of 3.5% and 1.0% with respect to the substrate, respectively. It is thus difficult to epitaxially grow a satisfactory single-crystal nitride semiconductor layer on a conventional Al.sub.2 O.sub.3 or SiC substrate.
Meanwhile, it is said that the lattice mismatches of LiAlO.sub.2 and LiGaO.sub.2 with respect to GaN are -1.45% and 0.19%, respectively. LiAlO.sub.2 has a crystal structure belonging to hexagonal system, and LiGaO.sub.2 has a crystal structure belonging to rhombohedral system similar to hexagonal system. However, the actual crystals of LiAlO.sub.2 and LiGaO.sub.2 have a locally distorted structure.
FIG. 21 shows a crystal structure of LiAlO.sub.2, and FIG. 22 shows that of NaFeO.sub.2. In the both of FIGS. 21 and 22, a larger white circle represents an oxygen atom and a smaller white circle represents a Li atom. A hatched circle in FIG. 21 represents an Al atom, and a hatched circle in FIG. 22 represents a Ga atom. As shown in FIGS. 21 and 22, the lattice sites of oxygen atoms form a shape distorted from an equilateral triangle.
The lattice constants of LiAlO.sub.2 and LiGaO.sub.2 corresponding to a lattice constant of hexagonal crystal of GaN is the distance between oxygen atoms. Tables 1 and 2 shows respective distances between oxygen atoms for LiAlO.sub.2 and LiGaO.sub.2, respectively. As can be seen from these tables, there are different distances between oxygen atoms in LiAlO.sub.2 and LiGaO.sub.2. Thus, it cannot be said that LiAlO.sub.2 and LiGaO.sub.2 are ideal substrate materials for GaN.
TABLE 1 ______________________________________ O-O Bonded with Al O-O Bonded with Li ______________________________________ Atomic Distance (.ANG.) 2.918 .+-. 0.004 3.301 .+-. 0.005 2.896 .+-. 0.005 3.296 .+-. 0.003 2.737 .+-. 0.003 3.430 .+-. 0.005 2.874 .+-. 0.004 2.737 .+-. 0.003 ______________________________________
TABLE 2 ______________________________________ O-O Bonded with Ga O-O Bonded with Li ______________________________________ Atomic Distance (.ANG.) 3.012 .+-. 0.006 3.189 .+-. 0.006 3.050 .+-. 0.003 3.050 .+-. 0.004 3.032 .+-. 0.005 3.182 .+-. 0.005 3.004 .+-. 0.006 3.251 .+-. 0.006 2.980 .+-. 0.004 3.222 .+-. 0.004 3.021 .+-. 0.005 3.360 .+-. 0.005 Average Distance (.ANG.) 3.016 .+-. 0.005 3.209 .+-. 0.005 ______________________________________
It cannot be said either that MgAl.sub.2 O.sub.4 substrate is an ideal substrate for GaN, since it causes a lattice mismatch of 11% with respect to GaN.